ISSN 08695938, Stratigraphy and Geological Correlation, 2011, Vol. 19, No. 5, pp. 502–514. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.V. Mitta, V.A. Zakharov, I.S. Barskov, V.B. Sel’tser, A.V. Ivanov, 2011, published in Stratigrafiya Geologicheskaya Korrelyatsiya, 2011, Vol. 19, No. 5, pp. 32–45. The Upper Bajocian–Lower Bathonian Boundary Section in the Outskirts of Saratov: Molluscan Characteristics and Biostratigraphy V. V. Mittaa, V. A. Zakharovb, I. S. Barskova, V. B. Sel’tserc, and A. V. Ivanovc a Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117647 Russia email: [email protected] b Geological Institute (GIN), Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia c Chernyshevskii Saratov State University, Astrakhanskaya ul. 83, Saratov. 410012 Saratov Received February 4, 2011; in final form April 13, 2011 Abstract—The analysis of all available data on the structure of the Bajocian–Bathonian boundary section in the outskirts of Saratov (Sokur quarry) and the taxonomic composition of its ammonites, belemnites, and bivalves revealed a continuous succession of the Pseudocosmoceras michalskii (Upper Bajocian), Oraniceras besnosovi, and Arcticoceras ishmae (Lower Bathonian) zones. In connection with the critique by Meledina et al. (2009), correlation of Bajocian and Bathonian boundary strata of the Central Russia and Northern Siberia is discussed. The inconsistency of Siberian bivalve and belemnite assemblages with Central Russian ammonite zones is explained by heterochronous invasions of different molluscan groups. Keywords: Saratov Volga region, ammonites, Bajocian, Bathonian, biostratigraphy, zones, Boreal–Tethyan correlation. DOI: 10.1134/S086959381105008X INTRODUCTION sediments in the Saratov–Volgograd region (Lower Volga River). A.V. Pavlov, a follower of Professor The Bajocian–Bathonian sequences of the Sara A.P.Pavlov, was the first to discover differences tov–Volgograd Volga region containing rare ammo between the Jurassic deposits of the Saratov region and nites of the family Parkinsoniidae have been studied typical Central Russia Jurassic sections, revealing for a century. Discovery of these Tethyan ammonites in members of light gray clays with intercalations of the quarry near Saratov (Mitta and Sel’tser, 2002) clayey siderite in the Saratov province, the socalled together with Boreal Arctocephalitinae (family Cardi “Upper Dogger with Parkinsonia parkinsoni Sow.” oceratidae), which had previously been considered to (Pavlov, 1904a, 1904b). Rzhonsnitskii (1905) estab be exclusively a highlatitude group, renewed interest lished the Dogger strata with Parkinsonia representa in this region. Recent years have produced new data tives below the Lower Callovian sediments with on the distribution of fossil remains in this section Cadoceras elatmae in the Chardym River basin (Sara (Mitta et al., 2004; and others). In addition, a bore tov Volga region). hole, which recovered older Bajocian strata, has been drilled in this area. Recently, Meledina et al. (2009) Arkhangelsky (1906) noted that in the Kamyshin published a paper with a critique of our views on the district of the Saratov Region, dark clays with clayey Boreal–Tethyan correlation of the Bajocian–Batho siderite concretions containing abundant Lower Call nian boundary beds. In this work, we discuss both ear ovian ammonites such as “Kepplerites aff. Gowerianum lier and recent data, and respond to our opponents. Sow., Cardioceras Chamussetti d’Orb., Cadoceras Voucher material discussed in this work is housed at surense Nik., C. Elatmae Nik., and C. Frearsi d’Orb.” the Borissiak Paleontological Institute (PIN) of the overlie pale sandy–micaceous platy clays with interca Russian Academy of Sciences and Vernadskii State lations of thinbedded micaceous sandstones contain Geological Museum (SGM) of the Russian Academy ing rare poorly preserved Pseudomonotis remains of Sciences. attributed to undivided Bathonian–Callovian interval. They are underlain by Bathonian layers separated in two members: HISTORICAL REVIEW (1) dark clays with gypsum and marcasite interca Until the end of the 19th century, the Callovian lated by beds and concretions of dark clayey siderites strata were considered to be the oldest Jurassic marine with Parkinsonia and abundant Pseudomonotis; the 502 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 503 lower clays enclose beds of ferruginous sandstone and described slightly later by P.K. Murashkin, who pro pebble gravel consisting of rounded fragments of Car vided the first image of ”Parkinsonia (?) mojarowskii boniferous limestones and cherts; Masar.” (holotype by monotypy). In addition, this (2) white calcareous sandstones, yellow quartz and author described Parkinsonia subcompressa sp. nov. gray micaceous–glauconite sands with beds of pebble (based on a single poorly preserved fragment) and gravel and separate large cherty blocks; the base of defined the genus Pseudocosmoceras with the type spe sands immediately above the Carboniferous lime cies Cosmoceras michalskii Bor., that was first stones is represented by red ferruginous conglomerate. described by Borisyak (1908) from the Donets Jurassic Rzhonsnitskii’s studies of 1904–1906 and 1914 (Lower Bathonian after Borisyak, 1917). This genus made it possible to specify the structure of Jurassic also included “varieties” of the type species (Ps. sections in the outskirts of Saratov. Below, we cite his michalskii var. minor and var. media and Ps. masarowici inferences concerning the lower part of the section: nov. sp. (with var. descendens, var. conjungens, and var. “The oldest rocks constitute the Bathonian Stage, inclara). Murashkin finishes his article with the words represented by gray and yellowish gray clays with inter “…from my point of view it is not possible at present to calations of dark gray siderite. These rocks yielded estimate the exact age of beds with the abovemen ammonites belonging to the genus Parkinsonia, tioned ammonites. It is conceivable that beds with bivalves of the genera Pseudomonotis, Pleuromya, and Parkinsonia subcompressa nov. sp., correspond to the others, belemnites, and wood fragments. The most zone with Parkinsonia würtembergica Opp. (? = Par abundant fossils are single Pseudomonotis species kinsonia compressa Qu.) correlated with the Upper resembling P. echinata Sow., which literally oversatu Dogger in the Jurassic section of northwestern Ger rate both clays and siderite beds. … The Bathonian many. One may confidently postulate synchronism of strata are immediately overlain by gray and yellowish Kamyshin Middle Jurassic sediments containing the gray sandy and micaceous clays with intercalations of abovementioned varieties of Pseudocosmoceras thinbedded dark gray finegrained micaceous sand michalskii from the Donets Middle Jurassic beds con stones, which are readily separated into fine plates. taining Pseudocosmoceras michalskii Bor. var. typica” These sediments are mostly barren of organic remains, (Murashkin, 1930, p. 159). In his table of “assumed while their upper part contains rare Lower Callovian phylogenetic relationships between examined ammo Cadoceras forms, which allow the entire sequence to nites” this author indicates the “Parkinsonia donezi be attributed with a certain degree of confidence to the ana Zone with hypothetical forms closely related to Lower Callovian. … These strata are overlain by black this species” and higher, beds with Pseudocosmoceras gypsumbearing clays with spheroid concretions and michalskii and Ps. masarowici, varieties included intercalations of dark gray siderite and intergrowth of (Murashkin, 1930, p. 150). ferrous pyrite. These clays contain abundant In 1930, Murashkin conducted fieldwork in the Cadoceras modiolare Sow., Cadoceras Elatmae Nik., Kurdyum and Chardym river basins (North of Sara Cadoceras Frearsi Nik., Macrocephalites sp., Cardio tov), where he noted “gray, locally, ferruginous sandy d’Orb., ceras Chamussetti Kepplerites Gowerianum clays with lenticular beds of siderites containing Par Sow., belemnites, Gryphaea, wood fragments, and kinsonia compressa var. close to var. würtembergica others” (Rzhonsnitskii, 1914, p. 60). Opp. emend. Nicolesco” attributed to the Bathonian In 1917, Jurassic sections in the Ilovlya River basin Stage (Murashkin, 1932, p. 74). were visited by A.N. Mazarovich, who noted under “Neocomian sands” in the Kamyshin district, thin In the middle of the 20th century, a group of Sara Oxfordian, “Callovian, problematic Bathonian, Bajo tov researchers headed by V.G. Kamysheva cian,” and underlying strata, which are divisible into Elpat’evskaya systemically investigated Jurassic fossils two units: the Karaulinskaya Group and Gnilushki in the Saratov Volga region. In several guidebooks and conglomerates and sands overlying the Carboniferous atlases (KamyshevaElpat’evskaya and Ivanova, 1947; limestones” (Mazarovich, 1923, p. 31). Mazarovich KamyshevaElpat’evskaya et al., 1956, 1959), these reached the conclusion that clays with siderite concre researchers described and illustrated (in most cases as tions underlying “clayey sands, siderites, and pale Bajocian) some Bathonian Parkinsoniidae. They had clays of the problematic Bathonian” are older, and limited material at their disposal (approximately attributed them to the Bajocian. He assumed that the 10 specimen), which prevented a comprehensive Bajocian Stage is represented by two zones: lower investigation of the taxonomic composition of the fos Stephanoceras humphriesianum